Sunday, October 8, 2006

Parasites in Fish, Part 2 -- Anisakis and Tapeworm

Now that we know all about cod worms, we are going to venture in the world of the more dangerous parasites: anisakis simplex and tapeworm. If you are cooking fish, you need not worry. According to FDA, you are 100% safe if the fish reached an internal temperature of 140F. Surviving the human intestinal track isn’t easy and requires that anisakis and tapeworm be at full strength. So, if you “only” raise the internal temperature to 120F, a parasite might survive (if he’s positioned in the middle of the fish fillet), but will be so weak that it will most likely die shortly after reaching your stomach.

Anisakis simplex is most common in fresh water and anadromous fish, like wild salmon, which are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. It is also common in certain small salt water fish, such as herrings and sardines. However, anisakis is rare in other salt water fish, such as tuna, swordfish and farm-raised salmon. Just like cod worm, it originates in seals. Tapeworm is mostly found in pacific wild salmon and fresh water fish. It originates in bears and land mammals. They are fascinating organisms and you can read all about the anisakis life cycle and the tapeworm life cycle on wikipedia.

Becoming a host to anisakis worms by eating them live can make you very sick---this disease is known as anisakiasis. According to wikipedia, its symptoms include violent abdominal pain, nausea, and vomiting. "Occasionally the larvae are coughed up. If the larvae pass into the bowel, a severe eosinophilic granulomatous response may also occur 1 to 2 weeks following infection, causing symptoms mimicking Crohn's disease." This isn't a life threatening disease (unless you have a weakened immune system) and is quite common in Japan where consumption of raw fish (that isn't previously frozen) is more wide spread. Unlike anisakis, tapeworms don't always manifest themselves with clear symptoms and can live in humans for decades if untreated, resulting in weight loss, Vitamin B12 deficiency, and potential anemia.

What used to bother me is the possibility of eating the eggs of these worms. Wouldn't they be too small for me to see, and what would happen if I eat them? Dr. Palm, from the Institute for Zoomorphology, Cell Biology and Parasitology in Düsseldorf, Germany, put my worries to rest by explaining the life cycle of these worms. While anisakis and tapeworm are in fish, they are in larvae form (not egg form). They can't reproduce until they find a mammal host (in the case of anisakis and cod worm, it has to be a marine mammal like a seal, so they can't reproduce in a human), and tapeworms rarely make it into humans.

Are you ready to swear of sushi yet? Not so fast. If you are a US resident, keep in mind that you live in a country that just threw away every single bag of spinach because of E.Coli threat. You don't think FDA would allow anything remotely dangerous to be served to the US public, do you? That's why FDA requires all fish with a potential hazard of parasites that is intended for raw consumption to be previously frozen. Freezing fish to -20ºC [-4ºF] or below for 7 days or -35ºC [-31ºF] or below for 15 hours will kill the parasites. Since the restaurants don't want to take any risk and want to avoid supply and demand price fluctuations, most go even further and freeze all fish (not only the ones that could be infected) before serving them raw. So, all that "fresh" sushi you've been eating is previously frozen.

What's counter-intuitive to most cooks is that farm-raised salmon is much safer to eat raw than wild salmon. Farm-raised salmon is served pellet food, which is ground-up, processed fish meat. Any parasites in the fish meat are killed in the processing and grinding stages. Since salmon only obtains dangerous to humans parasites via food, farm-raised salmon simply isn't exposed to them. So, next time you use salmon for gravlax, tartar, or sashimi, go for the farm-raised stuff. When the Food and Agriculture Organization of the United Nations tested various fish for parasites in 2003, no parasites were found in any farm-raised salmon species , whereas parasites were frequently found in wild salmon (section 5.1.4 of Huss et al., 2003).

Does salting fish like for gravlax or curing it in acid like for ceviche kill the parasites? Maybe. The salt or acid used for curing prevents bacteria from growing. It may also weaken or kill parasites. However, it’s not a full-proof method. Opinions in the scientific literature vary as to the degree to which salt/acid harms parasites. Most sources say that salting is more effective than curing in acid. Also, according to Dr. Gardner from Harold W. Manter Laboratory of Parasitology at the University of Nebraska, the acids in your stomach and intestines are at least as strong as lemon/lime juice. So, if you are making ceviche, I would suggest taking the same precautions as you would for eating the fish raw.

To put all this in perspective, the risk you take downhill skiing is an order of magnitude greater than the risk of eating raw, not previously frozen fish. Whether that risk is worth it is up to you. I hate downhill skiing and I love raw fish, so you can guess which risks I choose to take. In fact, the risk of driving or just walking down the street is probably higher than the risk of eating raw fish. I know plenty of people who were in life-threatening car accidents, and I am yet to meat a person who got infected by anisakis simplex or tapeworm. And let me tell you, I get way more pleasure from a bowl of sashimi than my morning commute.

Do parasitologists eat sushi in spite of their intimate familiarity with parasites? Both Dr. Palm and Dr. Gardner said “yes”. In fact, Dr. Palm just got back from Japan where he had really yummy not-previously-frozen sashimi.

What does all this mean to the home cook who wants to make sushi and ceviche? Buy your fish from a reputable source and use it that day if serving raw. Regardless of the parasite issue, fish that was not stored properly, or for too long, will grow bacteria and make you sick. Freezing fish that is not fresh will not help with the bacteria issue, but it will kill parasites. If you are not planning to freeze fish and want to eat it raw, I would limit your purchases to:
  • Large Tuna (Yellowfin/Ahi, Big-eye, Bluefin)
  • Hamachi
  • Branzino
  • Arctic Char
  • Scallops
  • Kampachi (farm-raised)
  • Farm-raised Altantic salmon (relatively low, but not insignificant risk of parasites)
If I find out about other fish that have extremely low occurance of parasites, I'll post them on Beyond Salmon.

What if you want to freeze your fish to eliminate even the slightest chance of getting sick from parasites? What’s the best way to freeze fish? Is all frozen fish equal? Can you buy frozen tuna from Trader Joe's, defrost it, and voila -- $5/Lb sashimi is served? In my next post, I'll answer all these questions and more.


H. H. Huss, L. Ababouch, L. Gram. Assessment and Management of Seafood Safety and Quality. 2003. Food and Agriculture Organization of the United Nations. FAO Fisheries Technical Paper 444.


Anonymous said...

Hi Joe,

I'm Helen's husband. The trouble with the web page you cite is that it's lacking in details.

"Salmon farmers may use pesticides and antibiotics to control outbreaks of disease among the fish."

Note the use of the word "may." As written, this statement is basically vacuous. If you're worried about pesticides/antibiotics, ask your fishmonger. They should be able to tell you whether such chemicals were used in the raising of Atlantic Salmon that you buy.

"The Environmental Defense has issued a health advisory for Farmed salmon due to high levels of PCBs and dioxins."

Anyone can issue a health advisory. The page that Monterey Bay links to doesn't provide any details. Where are the scientific studies showing that if you eat Atlantic Salmon regularly, something bad will happen to you? I don't see any. If you know of any, please send them along, I'd be very interested to read them.

"Waste from most farms is released directly into the ocean. Parasites and diseases from farmed salmon can spread to wild fish swimming near the farms."

Any fish or mammal that lives in the ocean releases waste into the ocean. This waste can pass along diseases and parasites. A valid concern is that a Salmon farm could create an unnaturally high concentration of waste. But, again, this depends on the farming company. Farmed Salmon are clearly better than wild Salmon in one respect: they don't transmit parasites that are dangerous to humans. Table 5.21 from the Huss/Ababouch/Gram report shows that such parasites are not found in farmed salmon, whereas they are quite prevalent in wild salmon.

Btw, all fish & mammals that live in and around the water have the potential to transmit parasites and diseases. It's misleading to say that farmed Salmon can infect wild fish, since if the Salmon had not been farmed, it would have also had the chance to infect (other) wild fish.

"And feeding farmed salmon actually uses more fish than it produces, which puts more pressure on wild populations."

This is also misleading. The same could be said about wild salmon---they eat more fish than they produce.

"In the market, there is currently no way to tell which salmon are coming from more-sustainable farms..."

I doubt this is true. I know that Whole Foods is quite particular about their farm-raised Salmon. While it's probably true that many fishmongers won't be able to tell you the details of how their fish was raised, that's largely because consumers don't ask.

Helen said...

Hi Joe,

Thanks for your question about farm-raised salmon. It's an issue that comes up over and over, and you've inspired me to do some research on the subject. There are two big questions: 1) does farm-raised salmon pose health risks? and 2) is it bad for the environment?

I have some information about the first issue. Here is an article from Tufts University School of Nutrition Science and Policy about PCBs in farm-raised salmon. At the moment, we have no data that PCBs in such small amounts are dangerous to your health. Here is my previous post on mercury in fish -- another hot button issue.

As far as antibiotics go -- Whole Foods label their fish clearly so that you know what you are buying, and they are starting to carry salmon without antibiotics. But keep in mind that most of the meat that you eat was treated with antibiotics too. Is eating farm-raised salmon every day a good idea? I don't think eating anything in such access is a good idea. But there is no scientific evidence that eating farm-raised salmon is harmful to your health.

From Monterey Bay Aquarium site: "When consumers eat this fish, the residues from the chemicals may affect their health or interfere with medicines they’re taking."

Some people are allergic to seafood all together. Does this mean we should consider it generally dangerous?

I haven't researched environmental issues yet. It's on my to-do list and I'll post about it as soon as I have some information.


Anonymous said...

Hi there,

Farmed Salmon have a VERY bad reputation environmentally, and farmed salmon may also possess significant health risks.

Here's some further information and some responses to your posts for further consideration.

Jason stated:
>As written, this statement is
>basically vacuous. If you're >worried about pesticides /
>antibiotics, ask your fishmonger.
>They should be able to tell you
>whether such chemicals were used
>in the raising of Atlantic Salmon
>that you buy.

Remember that the aquarium page is not providing an in-depth analysis of aquaculture. It is attempting to provide a summary of information that is widely available.

As for the antibiotics issue - there is very little chance that the average fishmonger is going to know if their fish is farmed or not. It is primarily specialty stores that spend the time to pay attention to that kind of food issue.

Jason stated:
>"The Environmental Defense has
>issued a health advisory for
>Farmed salmon due to high levels
>of PCBs and dioxins."
>Anyone can issue a health
>advisory. The page that Monterey
>Bay links to doesn't provide any
>details. Where are the scientific
>studies showing that if you eat
>Atlantic Salmon regularly,
>something bad will happen to you?
>I don't see any. If you know of
>any, please send them along, I'd
>be very interested to read them.

Health advisories are usually put out by agencies that have a duty to protect the public, so I think the comment that "anyone" can put out a health advisory is a bit weird. I can put out a health advisory, but no one is going to trust it! I would call the Monterey Bay Aquarium a fairly knowledgeable and trustworthy source!

Their stance is firmly backed up by the science. Here are some studies (out of hundreds available) about dioxins in farmed salmon:

To summarize the story told in these studies for you, the high levels of dioxins in farmed Salmon are a result of a process similar to bioconcentration resulting from the food that the farmed Salmon are fed. Pellets fed to farmed salmon are impregnated with fish oil made from small fish. The oil comes from the part of the small fish's body where contaminants are concentrated. Because a small amount of oil comes out of each fish, farmed salmon end up with a far greater amount of dioxin than wild salmon - and a disproportionate amount of fish are killed, taxing the environment that these fish come from. But because the small fish are not valuable for human consumption, we feed them to farmed salmon, which ARE valuable for human consumption - ignoring the environmental impacts.

Jason stated:
>"Waste from most farms is
>released directly into the ocean.
>Parasites and diseases from
>farmed salmon can spread to wild
>fish swimming near the farms."
>Any fish or mammal that lives in
>the ocean releases waste into the
>ocean. This waste can pass along
>diseases and parasites. A valid
>concern is that a Salmon farm
>could create an unnaturally high
>concentration of waste. But,
>again, this depends on the
>farming company.

Anti-aquaculture groups object to Salmon farming for many of the same reasons why factory farming is objectionable. In a farm situation, the animals or fish and held in large numbers within a comparatively small area. Because the fish/animals are so concentrated it is very hard to keep the space clean (so the fish/animals may be sitting in their own waste - but this is of less concern in the water) AND because they are so concentrated any parasite/disease will travel very quickly through the population.

Salmon farms put lots of food (overfeed) so that their fish get big. Extra food is also added because the pellets SINK - so there is a lot of wastage.

Remember that Salmon farms are an unnaturally large & dense population, so you get an exponentially higher level of parasites (captive breeding) and, bioconcentration, disease, et cetera.

This concentration of harmful factors would not be a big problem if it was dissipated by an ocean current, but most salmon farms are placed near streams and river mouths where salmon come to spawn. This means that more vulnerable members of the wild stock (like the baby salmon aka Salmon fry) are subjected to an unnaturally high concentration of parasites - so wild stocks become depleted.

This is why in factory farm situations, like Salmon farms, the fish/animals are often given antibiotics AS A MATTER OF COURSE. That is to say, they are given drugs even if they are not sick, as a preventative measure.

Wild salmon don't have access to the antibiotics that farmed salmon are fed, so they are far more vulnerable!

Jason stated:
>Farmed Salmon are clearly better >than wild Salmon in one respect:
>they don't transmit parasites
>that are dangerous to humans.
>Table 5.21 from the >Huss/Ababouch/Gram report shows
>that such parasites are not found
>in farmed salmon, whereas they
>are quite prevalent in wild
>Btw, all fish & mammals that live
>in and around the water have the
>potential to transmit parasites
>and diseases. It's misleading to
>say that farmed Salmon can infect
>wild fish, since if the Salmon
>had not been farmed, it would
>have also had the chance to
>infect (other) wild fish.

Ask yourself this: why don’t the fish have parasites? If wild fish have the parasites, and the parasites occur naturally… this must be because the fish farms give their fish antibiotics. So you are trading parasites for antibiotics in your food. Wild salmon have a lower population density, so they have less exposure to parasites... UNLESS they swim near a salmon farm.

Jason stated:
>"And feeding farmed salmon
>actually uses more fish than it
>produces, which puts more
>pressure on wild populations."
>This is also misleading. The same
>could be said about wild salmon
>- they eat more fish than they

This all goes back to the essential problem with all forms of factory farming:
- Farmed animals need to be fed something
- The food that farmed animals are fed must be taken out of the environment, placing an additional stress that is not there naturally

In this particular case, as previously stated, farmed salmon are fed food pellets flavoured with fish oil (they will not eat the pellets without the fish oil). The fish used to make the pellets are not useful to human, but that does not mean they don’t have a role to play in the local environment. Suddenly humans become a new predator, which impacts the balance of the overall ecosystem. If the food for the salmon was farmed as well it would be ISOLATED (somewhat) from the environment – but they are not, so there is a direct impact.

A secondary effect of a fish-farm: highly concentrated levels of decomposing matter in the local environment. Decomposing matter is eaten by bacteria, which use up oxygen. Lack of oxygen kills plants in the local area (not enough air to breathe), and you end up with “dead zones” downstream from fish farms.

Jason stated:
"In the market, there is currently no way to tell which salmon are coming from more-sustainable farms..."
>I doubt this is true. I know that
>Whole Foods is quite particular
>about their farm-raised Salmon.
>While it's probably true that
>many fishmongers won't be able to
>tell you the details of how their
>fish was raised, that's largely
>because consumers don't ask.

Totally true! But that's something people need to be educated about -they need to be told that they should ask. Or they can buy wild salmon and avoid the problem (not there are not problems with salmon fishing, there are environmental concerns there as well).

Helen stated:
>I have some information about the
>first issue. Here is an article
>from Tufts University School of
>Nutrition Science and Policy
>about PCBs in farm-raised salmon.
>At the moment, we have no data
>that PCBs in such small amounts
>are dangerous to your health.
>Here is my previous post on
>mercury in fish – another hot >button issue.

Again, a great point! However, this must be mitigated by the fact that PCBs are bioaccumulative. That is to say, PBCs NEVER leave your body and build up over your lifetime. Anything you can do to avoid PCBs is a good idea! The higher the concentration of contaminants in your body with free-chlorine (as with PCBs) the greater risk of cancer and many other health problems. PCBs are an issue with ALL seafood (and not an insignificant amount of other food), but are 20-30% higher in farmed salmon than in wild salmon. So if you really like to eat fish, you are better off with wild salmon.

Helen stated:
>As far as antibiotics go -- Whole
>Foods label their fish clearly so
>that you know what you are
>buying, and they are starting to
>carry salmon without antibiotics.
>But keep in mind that most of the
>meat that you eat was treated
>with antibiotics too. Is eating
>farm-raised salmon every day a
>good idea? I don't think eating
>anything in such access is a good
>idea. But there is no scientific
>evidence that eating farm-raised
>salmon is harmful to your health.

There is real evidence that farm-raised salmon is harmful to your health (as has been shows above); however, it is a matter of long-term exposure and degree. The overall environmental impacts may also be a moral concern for some.

But you are right, aside from the PCBs, farmed salmon are no worse than any other farmed meat product in terms of antibiotics.

Helen stated:
"When consumers eat this fish, the residues from the chemicals may affect their health or interfere with medicines they’re taking."
>Some people are allergic to
>seafood all together. Does this
>mean we should consider it
>generally dangerous?

No, but if something may contain nuts it has a warning label on it. Food with antibiotics which may interact with medications should have a warning label too, no? Just as products which "may contain fish" should probably have a warning label! :)

In any case, the point of all his is that there are real, significant, and important issues surrounding aquaculture, and suggesting farmed fish over wild fish is a very problematic recommendation. Joe’s criticism is certainly legitimate, as I have attempted to show.

Maybe this subject deserves for a blog post? Give the information to the people and let the people decide for themselves!

Best of luck, your blog is great!


Helen said...

Hi Steve,

Ok, you've inspired me. I will research this issue in greater detail and will publish a post on farm-raised salmon. Just give me a few weeks :)


Anonymous said...

I shall wait with bated breath!

Anonymous said...

Hi Steve,

Thanks for the reply and thanks for the link to the Hites et al. study. I can see why you might be concerned---the headlines certainly make it sound like farm raised salmon is dangerous due to levels of toxins which are much higher than the levels found in wild salmon.

"As for the antibiotics issue - there is very little chance that the average fishmonger is going to know if their fish is farmed or not. It is primarily specialty stores that spend the time to pay attention to that kind of food issue."

I could be wrong, but I'm fairly certain that even mainstream grocery stores in the Boston area (where we live) specify farmed/wild. Though, I'd certainly agree with you that few fishmongers would know whether farmed fish they sell was exposed to antibiotics/pesticides/etc.

Concerning the study: it is clear from the study that farmed salmon have much higher levels of PCBs/Dioxins/etc. than wild salmon. What isn't clear is whether these levels are sufficiently high to pose a danger. Also, the wild salmon on average have lower levels of omega-3's, so even if the PCB/Dioxin/etc. levels are high enough to pose a danger, the advantage of lower levels of PCBs/Dioxins/etc. might not be sufficient to offset the disadvantage of lower levels of omega-3's.

Yes, researchers who published the original Jan 9, 2004 Science paper have done later nutritional studies, e.g. J. A. Foran et al. "Quantitiative Analysis of the Benefits and Risks of Consuming Farmed and Wild Salmon". However, they do not provide evidence that wild salmon is better for you than farmed salmon. In particular, they do not study the change in expected lifetime (or other measure of healthfullness) from switching salmon consumption from farmed to wild. Yes, wild salmon provides more omega-3's "per unit risk" (Figure 1). However, consumers don't eat food "per unit risk." The amount of salmon a consumer eats is unlikely to change in switching from farmed to wild. Also, the "unit risk" discussed by the authors seems quite small. They define unit cancerous risk as a 0.001% increase in the risk of death from cancer. If a consumer eats enough farmed salmon (purchased from, say, Eastern US retail locations) to achieve the recommended omega-3 intake (1 g/day), he/she would face merely a 0.0125% increased risk of cancer (due to the PCBs/Dioxins/etc.) according to their calculations. This might be more than offset by the decreased risk of early death from coronary heart disease (versus eating wild salmon instead). They define unit non-cancerous risk as the U.S. EPA Reference Dose (RfD). The U.S. EPA RfD is "a scientific estimate of a daily exposure level that is not expected to cause adverse health effects in humans." I.e. it is a level many times lower than the level at which adverse health effects have ever been observed. Consumption of sufficient (Eastern US retail) farmed salmon to provide 1 g/day of omega-3's does not exceed the RfD.

"Anti-aquaculture groups object to Salmon farming for many of the same reasons why factory farming is objectionable. In a farm situation, the animals or fish and held in large numbers within a comparatively small area. Because the fish/animals are so concentrated it is very hard to keep the space clean (so the fish/animals may be sitting in their own waste - but this is of less concern in the water) AND because they are so concentrated any parasite/disease will travel very quickly through the population."

It sounds like this isn't so much a concern with the general idea of farming, but rather with farms which focus on lowering costs to the detriment of the environment and animal/fish health. Personally, I'd like to see designations for more environmentally-friendly fish farming. It seems that such designations on other products (e.g. "organic") have helped gradually improve the quality of farming in the U.S. via consumer choice.

"Salmon farms put lots of food (overfeed) so that their fish get big. Extra food is also added because the pellets SINK - so there is a lot of wastage."

Do you know what is the typical degree of wastage?

"This is why in factory farm situations, like Salmon farms, the fish/animals are often given antibiotics AS A MATTER OF COURSE. That is to say, they are given drugs even if they are not sick, as a preventative measure."

Are you certain that all salmon farms use antibiotics?

"Ask yourself this: why don't the fish have parasites? If wild fish have the parasites, and the parasites occur naturally... this must be because the fish farms give their fish antibiotics."

I think you missed my point. The main parasites which are dangerous to humans (Table 5.21 from Huss/Ababouch/Gram) are transmitted via food. The grinding process in producing pellets for farmed salmon kills any such parasites (antibiotics have nothing to do with it).

"Decomposing matter is eaten by bacteria, which use up oxygen. Lack of oxygen kills plants in the local area (not enough air to breathe), and you end up with "dead zones" downstream from fish farms."

Don't plants need CO2 to "breathe", not O2?

"That is to say, PBCs NEVER leave your body and build up over your lifetime. Anything you can do to avoid PCBs is a good idea!"

There is nothing you can do to completely eliminate exposure to PCBs. They can be found in most, if not all, foods. The important question isn't "how do I eliminate PCBs from my diet?", but rather "how do I avoid exposure to unsave levels of PCBs?" As far as I can tell, neither the original R. A. Hites (Science, 2004) study, nor follow-up studies indicate that farmed salmon have unsafe levels of PCBs.

"There is real evidence that farm-raised salmon is harmful to your health (as has been shows above); however, it is a matter of long-term exposure and degree."

Why do you think so?

Anyway, I hope my comments were constructive. My goal isn't to push one angle or the other, but rather to understand the real underlying concerns.



Anonymous said...

One concern that I think has not come up in this discussion is the ecological consequences of farm-fish escapees. In areas where Atlantic salmon are farmed and are native (e.g., east coast of Canada, Maine, Norway) there is concern that farmed fish could escape, interbreed, or out-compete wild populations of Atl salmon. Interbreeding farmed and wild fish can kind of "dumb down" the genetic make-up of wild populations.
However, on the west coast where Atl salmon are farmed (British Columbia and Washington), this is an introduced species. Our inland waters have been devasted by introductions of non-native fish. Many, many, many examples. There are all kinds of concerns when it comes to introduced species-- including passing diseases and parasites, i.e., those normally only found in Atlantic fish, to our native fish.
There was also some talk about wastage/efficiency in food that is fed to farmed fish (with respect to lbs fed to lbs grown). I'm not sure if this was brought up, but one of the problems is that we're already over-fishing wild fish stocks. Catching even more fish to grind up for food only compounds the over-fishing problem.
Oh yeah... and wild fish just taste better. :)
Thanks for this forum.

Anonymous said...

From your wiki link on Anisakis:

A disclaimer regarding the absolute safety of properly cooked fish:

"Allergic reactions

Even when throroughly cooked Anisakis pose a health risk to humans. Anisakids (and related species such as the sealworm, Pseudoterranova spp., and the codworm Hysterothylacium aduncum) release a number of biochemicals into the surrounding tissues when they infect a fish. They are also often consumed whole, accidentally, inside a fillet of fish.

People who are sensitized to nematodes can have severe anaphylactic reactions after eating fish which have been infected with Anisakis spp. This is often confused with a fish or shellfish allergy, as the allergenic components of Anisakids are difficult to test for and often produce a reaction in tests for other allergens."

Anonymous said...

Just a quick note on antibiotics in animal feed - almost ALL antibiotics given to meat producing animals (mammals and poultry ) are in non-therapeutic doses. In other words, they are not fed enough to prevent or treat disease. The antibiotics are in the feed because low levels of antibiotic in feed increases the weight (yield) from the animal. The major concern in the use of low dosage antibiotics for increased yield is that these low levels can increase the incidence of antibiotic resistant bacteria all the way through the food chain. This isn't environmentalist propaganda - just ask any farm vet.

Anonymous said...

Just had to weigh in to mention that antibiotics and parasites have nothing to do with each other. A previous comment said that you don't find parasites because of antibiotics. Note: the parasites we're talking about are not bacteria. Antibiotics would likely have no effect on these parasites.

Anonymous said...

Hi, I just got into making sushi. I used to buy them but the cost is pretty high. Also if I make them I can put whatever I like so that's a big plus. In your Blog you said farm raise Salmon(atlantic) is better for sushi than wild. As I see the Salmon at my local supermarket are farm raise Canada. I was wondering is there a differences between Canada and Atlantic farm raised? Please reply I'll be checking everyday for your answer. Thank you for your wonderful blogs it really gave me alot of tips and help for my sushi making. Hope you can help thank you so much.

Helen said...

Hi Anonymous,

If I understand correctly, some of the "Atlantic" salmon is farm-raised on the west cost. "Atlantic" is a species, not a farming location.

The reason that farm-raised salmon is safer for sushi is that its feed is controlled, so the occurrence of parasites that can harm you is tremendously lower. The salmon I used for sushi is also from Canada (if that makes you feel better :)


Anonymous said...

Great, I do feel alot better. But however I wasn't doing any research when I was making Sushi. I never froze my fish do you think I'll be ok? I was curious though the Korean supermarket that I bought the Salmon from they told me not to buy the fresh farm raise Salmon. They said that it wasn't sushi Salmon. Then I went to another Korean supermarket that was selling the same thing. One of the guy that was working there told me it's ok to make sushi with. So why is that so? I tried to ask the guy that told me not to buy it because it wasn't sushi salmon but he doesn't speak engish well so I couldn't get any information from him. i like to know about the freezing the salmon. so i read the blog you wrote. you said freeze the salmon for 7 days then put it in the fridge for 24 hours. so within the 24 hour in the fridge it should defroze? thank you so much for your fast reply and thank you for help.

Helen said...

yes, you put it in the fridge for 24 hours so that it can defrost. Defrosting it at room temp is a no-no in general, but particularly for serving it raw. "Sushi salmon" that market was talking about just means that they did the freezing and defrosting for you :) When buying fish for sushi, I wouldn't go to a place that doesn't speak the same language you do. The fishmonger should be comfortable answering all your questions clearly. Also, keep in mind that it's best to go to the absolutely best fish market in the city with the biggest turnover. Don't be surprised if they'll have some of the highest prices in the city. There is no such thing as a free lunch when it comes to raw fish, but even the priciest fish market will result in very affordable sushi when you make it yourself. For example, I normally pay $20-30/Lb for most of the fish I serve raw. But when you take into account that 1 Lb serves 2 people more generously than any sushi restaurant (normally you get 1/4 to 1/3 Lb per person), $20-30 can get you a very nice sushi meal for 2 at home.


Anonymous said...

About the worm, if the salmon i ate had some and is in my body now what can i do about it? is there a cure or the only option is go under the knife? i read a blog about a guy got a 12 inches worm or was it feets i forgot. but that really freaked me out though i'm not going to stop making sushi though :-) do you know a percent about people getting sick from farm raise salmon that was ate raw that's in the USA? i hope i'm not annoying you with all these question but you're my only resource. thank you so much.

Helen said...

please, stop worrying :) The chance of you getting a parasite from raw farm-raised salmon is way lower than getting salmonella by eating a salad at some fast food joint. So, relax. Are you feeling ok? If you are, stop worrying. To make you feel better, the incidents of food-born illness from raw fish in US is 1 in 1 million serving and from chicken is 1 in 25,000.

Anonymous said...

well, see i don't know is it from reading the blog or i'm just worrying too much. because you stated how you should feel and signs when u have it. i do have abdominal pain, haven't vomit yet though. well i just hope i'll be ok hehe. thank you so much for your help and your time.

Anonymous said...

Hi, I was wondering do you make sushi with flying fish roe? I bought a box (half pound) I notice in the crowded box full of eggs I see some string like tat is about half an inche. the color is clear to alittle light orange color. It doesn't move but I was wondering what that is? Do you have any information on it? About parasite or bacteria is there any in the flying fish roe? Hope to hear from you soon thank you.

Helen said...

you can definitely have bacteria grow in fish row, just like it can grow in all fish, meat, chicken, vegetables, etc. When the decompose, bacteria starts to grow. They are microscopic organisms and you can't see bacteria with a naked eye. Parasites are another matter. I don't know if it's common to see a parasite (worm) in row, though it's feasible. Ask the place where you get the row. If they are not knowledgeable about their products and possible risks, it's probably not a good place to buy fish for serving raw.


Anonymous said...

But do you know what those string like things are? there isn't many just like in the whole half a pound only about 5 at most but they are clear sometimes hard to see as well. You know what those are? If you make sushi with roe I think you should've seen them before. but yes i'll try the place I bought it at and see if I can get any information about it thank you.

Anonymous said...

THe one thing about farm raised salmon is if they do get parasites (which they do) they become very rampant) its no different than factory farmed cows, they are more stable in flavor and cost but if one gets sick they all get sick, and if something goes wrong when butchering its a big problem.

Helen said...

If I understand correctly, parasites (at least the ones that are harmful to humans) are not contagious. They are not a disease. Just because farm-raised fish are in close contact with each other, doesn't mean they are more prone to spreading the parasites that are of concern to you. Sea lice are a different problem. They can spread in a farm, but they have nothing to do with the risks of eating fish raw.

Anonymous said...

I was eating some cooked Salmon the other day. i looked at the meat as I was eating and I noticed that there was a small white "coil" in the meat. upon further inspection, I discovered that these "coils" were through out the entire piece. I stopped eating it. I have never noticed this before. Does any one know what it could be? I'm thinking of bugs and I don't like to eat bugs. help!!

Helen said...

Hi Anonymous,

Yes, quite possibly those were worms, but if the salmon was cooked, they don't pose any risk to your health. Of course, they are still yucky, so I don't blame you for throwing the salmon away. What type of salmon was it? Farm-raised Atlantic, king, coho, sockeye?


Anonymous said...

What about fresh caught lake trout? I love to camp and fish and have always considered making sushi while I'm out there. Is it possible to see (and remove) the potential parasites if the fillet is held up to light?

Helen said...

I would strongly advise you against eating any lake trout raw without freezing it for at least 7 days (this kills parasites). Parasites in raw fish are very common. Even experienced sushi chefs can't always get all of them out. I personally wouldn't trust myself and I teach sushi classes :)

megan said...

I was eating cooked Wild Copper River Salmon this evening and discovered a thin reddish brown one-inch long dead worm. I was wondering if anyone knows what it could have been?

Helen said...

Hi Megan,

It was most likely cod worm, but if the fish was cooked, cod worm and any other parasites pose no health risk to you.


Anonymous said...

YUCK YUCK YUCK...I just bought wild sock eye salmon from Publix. I usually get farm raised, but the wild salmon is in season and on sale so I thought I would get some for a change. I picked it up to put it on the grill and there was a worm on top...gray and very thin, curled up, with white on the very end...Im NEVER buying wild salmon again. Especially after reading this article.

Anonymous said...

i don´t understand this:

is tapeworm present in farmed salmon?

can a fish be checked somehow?

what is the risk of getting tapeworm infection?

honestly, i am a bit worried right now...

thank you for your answers!

Helen said...

tapeworm is not present in farmed salmon (at least not normally). if you are cooking the fish, stop worrying. even if it has the tapeworm, cooking will kill it. if you are eating it raw, that's another story.

Anonymous said...

You should all know that I USED to make my own gravlox from salmon, but don't anymore because I have had the fish roundworm from eating my homemade gravlox--from farm raised fish. This was cured for 3 days with a large amount of salt, sugar, and spices. I had NO symtoms of infection until a 10 inch worm came out when using the bathroom. I went to a doctor immediately and was treated. The doctors recommend NEVER eating raw fish--but I did not ask them about freezing the fish first. However, I will NEVER eat raw or cured fish again, as the experience is so unpleasant if you get a parasite infection. I would much rather go downhill skiing and break my leg! Raw uncured fish is even more likely to contain parasites than cured or smoked fish--even farm raised fish. And please do not tell people that if they have no symtoms, they are fine. Some signs of parasite infection are so mild at first that you easily dismiss them. Please don't make sweeping comments about raw fish consuption--according to the CDC, there is ALWAYS some risk of illness unless fish or meat is cooked.

Helen said...

Hi there,

First of all, I am terribly sorry to hear about your parasite infection. I am sure it was extremely unpleasant and I understand your reluctance to ever ear un-cooked fish again.

That being said, I think it's important to realize that there is always some risk associated with eating. To some people the risk is worth it, to others it's not. I never claim that raw fish is 100% safe to eat. Nothing is. People who don't want to take any chances should eat NOTHING raw. Remember the e.coli in spinach scare? Sure it happens, but should we suggest everyone stops eating spinach salad and only consumes spinach cooked?

To put things in perspective -- the worms have a huge ick factor, but compared to e.coli they are way less harmful.

Another thing I can suggest you look into (if you haven't lost all interest in curing fish), is using arctic char. It is farm-raised inland (from what I understand), and should have way less incidence of parasites than salmon. I am not guaranteeing its safety, but you might want to do a little research and decide if the risk is small enough to be tolerable (or maybe not ;)


Ginger said...

seems that Jewish people are not allowed to eat these worms for religious reasons,
see this video..
move it to the 2 minute mark

Anonymous said...

Can a doctor test for the presence of tapeworm in the body of someone who is not symptomatic of one? I had some fresh samon that I may have slightly undercooked (done with the exception of some slightly darker pink flesh in the middle). Am I just being a worry-wort?

Helen said...

most decent restaurants will undercook their salmon :) Yes, doctors can test you for tapeworm. I personally wouldn't worry about it, but I am not a doctor.

DrO said...

Interesting blog -- thanks! I eat all of my fish cooked (see below), because I would only trust a sushi/sashimi chef in Japan.

Cases of fish tapeworms do occur. Around 1988, one of my freshman biology students came to my office after my "invertebrates" lecture, during which I always discussed trematodes, cestodes (flatworms) and nematodes (roundworms). I had told my students you can sometimes see tiny roundworms or the proglottids of tapeworms in fecal material (e.g., your own or your pet's). I also told them how to detect pinworms in young children.

Apparently, the next time the student had a bowel movement, she looked in the toilet and saw lots of proglottids. She came to see me the day after, because she was understandably upset about it. I helped her re-trace her steps to figure out where she could have picked up a tapeworm-initially I thot it most likely she had picked up a pork tapeworm by eating a pulled pork sandwich, e.g., at a county fair. However, the only thing that made any sense was that she had recently been to New York City, where she had eaten sushi for the first time. So, I advised her to consult a medical doctor to see if she might have a fish tapeworm.

After that, I didn't see her for about 3 weeks. When she finally came back, she said she indeed had fish tapeworms and when the doctor gave her the anthelmintic drug, she hemmorhaged internally and ended up in the hospital. The hospital doctor said the tapeworm(s) had probably been killed too quickly(?), which resulted in the intestinal bleeding (on top of the anemia she had from having tapeworms in the first place). She said the hospital doctor said he would have recommended a lower initial dose of the anthelmintic (to kill off some worms?), followed by a higher dose later on. That way, she wouldn't have had so many perforations in her intestinal lining at once. I am not a medical doctor, and this is only second-hand information. I did ask my student to tell her regular doctor about this so that he might know how to better treat patients for intestinal tapeworms in the future.

I should add that some years ago I read a note in Science News about a surgeon who had to resort to exploratory surgery because he couldn't figure out what was wrong with a patient who frequently ate sushi. As he was doing the surgery, he saw some pink trematodes creeping across his glove, which turned out to be a taxonomically unknown species. The implication was that it wasn't the common, Nanophyetus salmincola. However, I've tried to Google about this to get more info about the supposedly unknown parasite, to no avail.

Helen said...


Thanks for the info about tapeworms. I am wondering why you might suspect a pulled pork sandwich? In order for pork to shred like that, it has to be cooked for hours and will be at least 180F inside (most likely around 200F). Wouldn't tapeworm die at that temperature?

About sushi -- most sushi served in restaurants has to be previously frozen. From what I understand, parasites can't survive in such cold temperature.


Ruth Orme-Johnson said...

Hi Helen,

Thank you so much for this insight. I joined a CSF (Community Supported Fishery) this year, buying a seasonal share to split between myself and a friend. Today, however, she found herself filleting one of the fish from our share to discover two quarter sized "nests" (i don't really know what you'd call them) of worms. One was in the meat itself, the other in the body cavity. The question we have is, how many worms are too many worms? I asked the fishery, but they haven't responded yet, and I'm wondering if I should venture on and try this for dinner tonight.


Ruth Orme-Johnson said...

Dear Helen,

So I have a question about how many worms may be too many worms in one's fish. I'm wondering because a friend of mine filleted a fish for us to share, and found two quarter sized "nests" of worms in the fish (one in the body cavity the other in the muscle). Do you have thoughts about whether or not that's okay? Let me know, thanks!


Helen said...

Hi Ruth,

It's ok safety-wise assuming you are cooking the fish to 140F, but it's very unappetizing :(

Report it to your CSF and see if they'll give you another fish next week.


Anonymous said...

I got a parasite from shrimp tempura rolls. It was baffling--but I know its what I had. On top of my rolls was the salmon fish roe--and I think that's how I got it. I was violently ill for two months. Nausea every day and diarrhea and horrible abdominal cramps after every meal. I don't think I'll ever eat any sushi roll that I don't make myself again--and it will have nothing raw on it. A shame because I used to love love love my sushi rolls. It has to be a lot more common than people think. I underwent so many tests because I didn't know what was wrong and no tests knew to test for Anasakis which is what I think I had. I could go into more grosser detail here but I won't--suffice to say I KNOW it was a worm. Horrifying.

Helen said...

So sorry about your terrible experience. Hope you are doing better now :)

jen do said...

Hi, nice to e-meet you. I got anisakaisis from consuming raw salmon sushi at a restaurant in Seattle. Caused a complete bowel obstruction. Terrible pain. Had to have surgery to remove a segment of my small bowel.

I still eat salmon sushi and I also downhill ski :)

Has some raw farm-raised from the grocery store last night and it was delicious!

Thanks for your informative article.


Helen said...

Wow, Jen -- you are a brave woman. So sorry about your terrible experience with anisakis.

elizabeth said...

So I've been all over google trying to find out what was in my (Whole Foods) cooked Alaskan Salmon that I just ate for lunch. The reason I'm worried is because I found a tapeworm in my (Whole Foods) salmon a few weeks back that me and a friend were ABOUT to eat raw! Anyways, I now cook all my salmon because I like getting wild, non-pesicide/antibiotic farm-raised (just to be safe) salmon, but for lunch I cooked a couple fillets and found long, hard/tough, white strings running perpendicular to the flesh and I'm wondering if that was a tapeworm infestation. It was only in one part of the 3 large fillets I cooked but it seems unlikely that the worms would stay stretched out long like that and turn tough like bones or tendons. Do you happen to know if this is how their bodies react once being cooked? I sure hope not because I just ate around them. :) Salmon is soooo good for you!


Lena said...

Dear Helen,

I really got concerned after reading all the comments… I’ve been happily eating sushi for the past 13 years after I moved to US and even made some myself… Like you said, I need to trust the source. However, recently, I started to make my own version of “smoked” salmon w/o smoke. Just salmon covered with salt and sugar for a couple of days. I usually get fish in the Fresh Market (not sure you have same chain in Boston) and don’t freeze (well, now I will!) A couple of questions:
- will salting process kill bacteria?
- if fish has bacteria – would it smell “strange” ?
- if I slice salmon really thin – should I be able to see a worm?
- I already made my salmon w/o freezing first…hate to throw away food. Is it a good idea to freeze it after it’s been salted to kill parasites? (eh.. probably not).
Thanks in advance.

Helen said...

Hi Lena,

Here are some answers:

- will salting process kill bacteria?

yes, but it depends on how much salt you use, how long you cure, and how fresh the fish was to begin with. as long as there was no off smell before you cured it, I wouldn't worry about it.

- if fish has bacteria – would it smell “strange” ?
yes, if your fish didn't smell, it's most likely fine.

- if I slice salmon really thin – should I be able to see a worm?

You'll be able to see cod worm and anisakis if you know what to look for. the real danger is tapeworm and you won't see that. If your salmon is farm raised, I wouldn't worry about it (it can't have tapeworm). if your salmon is wild, tapeworm can be an issue and I would strongly recommend you freeze the fish.

- I already made my salmon w/o freezing first…hate to throw away food. Is it a good idea to freeze it after it’s been salted to kill parasites? (eh.. probably not).

yes! freezing after curing is perfectly fine. I would only bother to do that if you are using wild salmon.

Generally speaking, this is all pretty safe stuff compared to downhill skiing (and probably even driving :)


Lena said...

Thank you so much!!!
I think it was farmed salmon - it's more pink than red. I don't ski... broke many bones while ice skating back in Moscow :)

Anonymous said...


Thank you for a very informative article. Would I be correct in assuming that raw scallops, prepared in acid, may contain certain forms of bacteria, but not parasites? In other words, I do not think the three forms of parasites mentioned in your article can be found in uncooked scallops.

Thank you!



Helen said...

Scallops are safe. They don't host the 3 parasites that can be harmful to you. All you need to worry about is them being very fresh and not treated with preservatives. In other words you want "dry" scallops.

Anonymous said...

Correct me if I'm wrong, but to my knowledge "farmed" fish are raised in open pens that float in the water. These pens can be kept in the the ocean, in fresh water, really anywhere. So to say that farmed fish cannot have worms seems absolutely preposterous to me. If a fish, that the farmed fish feed on, happens to find it's way into the one of the pens somehow, it is definitely possible that the farmed fish will get a parasite. I usually slow cook salmon at 250 degrees and I was just searching to find out what temperature the middle needed to be when I found the article / site...

Helen said...

It's true that salmon is farmed in the ocean and if some contaminated shrimp wonders into the salmon cage, salmon can get contaminated if it eats that shrimp. But, you have to think about the odds here. These salmon are not relying on wild feed. They are being fed pellet food, so the odds of them getting parasites is puny compared to wild salmon. Another thing to understand is that parasites are not a virus. If one fish gets it, doesn't mean the rest of them get it too. When the fish get parasites, they are mostly restricted to their gut. In a few instances they might get into the flesh and get picked out by the fishmongers. The odds of getting a parasite in farm-raised salmon is very low. Is it zero? No. But few things in life carry zero risk. How does it compare to driving? It's a laughable risk compared to driving. If you are planning to drive to the store to pick up farm-raised salmon, you are not changing your risk much by cooking it to less than FDA suggests. If you want to cook salmon to a completely safe temperature, it will be very unpleasant (140F). When I slow cook my salmon, I cook it to 115-120F.

Wild salmon is a completely different story. The risk of them being contaminated is quite large.

If you want to reduce the risk close to zero for both farmed and wild salmon, freeze them for 7 days.

Anonymous said...

Thanks Mrs.Rennie. Your research is very helpful and I'm glad to hear about the information from a well known trusted source like yours. Thank you again.